Proximity sensors are used to detect the position or location of an object. Various types of proximity sensors are available, including optical, inductive and capacitive sensors.
Optical proximity sensors can employ reflective techniques, for example, to detect the presence or absence of an object in the vicinity of the sensor. A typical technique is to use a light emitting diode (LED) and an optical detector configured in such a way that light emitted from the LED is reflected from an object back to the detector. The light source can be selected so that the light is suitable for detection by the light detector. Thus, for example, the light source can generate light of a frequency that the light detector is best able to detect and that is not likely to be generated by other nearby sources.
In designing small optical proximity, issues of electrical interference, optical crosstalk, and signal-to-noise ratio often need to be addressed. For example, there are various potential sources of optical interference in an optical proximity sensor: external (e.g., sunlight, indoor lighting, unintended targets) and internal (e.g., optical crosstalk between the subcomponents of the proximity sensor). External interference sometimes can be suppressed or reduced as part of signal processing. Internal interference, on the other hand, can be more challenging to manage, particularly in applications where the proximity sensor is mounted behind a transparent or semi-transparent cover such that the intensity of the light reflected from the cover may be similar in magnitude to the signal of interest.